The invention relates to a pneumatic pad made of a rubber material which is hot-vulcanized in a press.
Prior art pneumatic pads are made of a rubber material which is vulcanized in a hot press. Such a pad has a rectangular shape and incorporates an airtight envelope arranged inside of, and connected to, a connection device positioned in a corner and accessible from outside.
The pad has a first layer of a cord and special steedcord reinforced flat rubber material arranged outside this envelope, the direction of reinforcement running parallel to the second side of the rectangle. The layer has a rectangular cut having a width which substantially equals the length of the first side of the rectangle, and a length which is not essentially shorter than the double length of the second side of the rectangle. The layer is bent around edges running parallel to the first side of the rectangle.
The pad has a second layer likewise of a cord, especially steelcord reinforced material arranged outside the first layer, with the direction of reinforcement running parallel to the first side of the rectangle. This layer exhibits a cut of rectangular shape having a width which essentially equals the length of the second side of the rectangle and a length which is not essentially shorter than the double length of the first side of the rectangle. The pad has turned-under edges running parallel to the second side of the rectangle. So that the turned-under regions of this second layer are on the other side of the pad opposite the turned-under regions of the first layer.
The pneumatic pads disclosed in U.S. Pat. No. 4,372,533 have a cut for a first and second layers, the length of which is chosen in such a way that the turned-under edges overlap in each case. U.S. Pat. Nos. 4,067,544 and 4,143,854 also show this feature. The overlap of the pad shown in U.S. Pat. No. 4,372,533 is extended so far that the overlap area is greater than 75% of the total area of the rectangle defining the pads. The cord-reinforced flat rubber material is similar to the continuous weblike rubber material used in the tire industry and is reinforced by fabric. The reinforcing cords only run in one direction through the fabric (the beam), while the other direction is constituted of threads of lesser tensile strength. This reinforcing fabric is embedded in an initially unvulcanized rubber material.
During the production of prior art pneumatic pads by hot-vulcanization press molding, the unvulcanized inner envelope and unvulcanized two outer layers are assembled together as described above and laid into the press. With the step of hot-vulcanization, the overlapping regions of the envelope combine and form an airtight arrangement. At the same time, the contacting surfaces between the outer surface of the envelope and the inner surface of the first layer, as well as the outer surface of the first layer and the inner surface of the second layer, enter into an intimate bond. During this step, care has to be taken that the bond extends over the entire area of the pad. Any air enclosures or bubbles between the envelope and the first layer and between the first and the second layer must be carefully avoided by appropriate means. Due to the high loads a pneumatic pad experiences in practical operation, enclosed air bubbles will become compressed and force an at least partial separation of joined surfaces, i.e. split the pad. Enclosures of air must therefore be avoided during the manufacture of any pneumatic pad and especially before the step of hot-vulcanization is performed.
In the pad or cushion of the U.S. Pat. No. 4,372,533, the cuts for the first and the second layer overlap as described above. The thickness of the pad abruptly changes at the end sections of these overlaps. Thus, a danger exists that slender, lengthly air compartments are formed at the two edges of the cut running crosswise to the direction of reinforcement, which may initiate the tearing apart or cracking mentioned above. To inhibit this cracking, small strips which are not further designated or described are inserted. The task of these strips evidently is to avoid a sharp step within the side faces of the pad. The strips are positioned in such a way on the edges of the two layers, that they extend the overlapping regions to the change-over at the edges of the pad. Four additional strips are necessary which have to be positioned with care during the manufacture of the pad. Even if this operation is performed with care, gaps are inevitably formed and are left open between the additional strips and the edges of the first or second layer. Such gaps are already visible in the figures of the U.S. Pat. No. 4,372,533. The additional strips do not abut flush against the edges of the cuts.
The known prior art pads consequently have the disadvantage that four additional strips are needed during the manufacture, and these strips have to be positioned specially and carefully. In spite of this expenditure, as described above, no definite security is reached that no air enclosures appear within the walls of the pad.
It is an objective of the invention to avoid the disadvantages of the known pneumatic pads, and to improve this pad to the extent that the individual plies or beds establishing the pad are so joined that virtually no air enclosures can appear.